1. Field of the Invention
The present invention relates to a frequency conversion apparatus having an improved mixer, and more particularly, to a frequency conversion apparatus having an improved mixer compensating for phase miss-matching of two intermediate frequency (IF) signals having a 90 degree phase difference, improving a signal-to-noise ratio (SNR), and increasing an image-rejection ratio.
2. Description of the Related Art
Generally, a receiving structure, such as a zero-IF structure or an image-rejection structure requires an in-phase (I) signal and a quadrature-phase (Q) signal according to characteristics of the receiving structure. Miss-matching in phase occurs in the I and Q signals due to various factors and lowers a signal-to-noise ratio, thereby reducing receiving sensitivity of the receiving structure.
FIG. 1 is a block diagram of a conventional frequency conversion apparatus used in the zero-IF structure or the image rejection structure. Referring to FIG. 1, the frequency conversion apparatus includes a local oscillator 11 generating an oscillating frequency LO signal, a phase shifter 12 shifting a phase of the oscillating frequency signal LO by 90 degrees to generate a first oscillating frequency (LO) signal LOI, an I-mixer 13 mixing the first LO signal LOI with an RF impact signal RFIN to generate a first intermediate frequency signal (IIF), a Q-mixer 14 mixing a second oscillating frequency (LO) signal LOQ with the RF input signal RFIN to generate a second intermediate frequency signal QIF, and filters 15, 16 low-pass filtering the first IF signal IIF of the I-mixer 13 and the second IF signal QIF of the Q-mixer 14, respectively.
The mixers 13, 14 of the conventional frequency conversion apparatus receive the first LO signal LOI and the second LO signal LOQ having a 90 degree phase difference with the first LO signal LOI from the local oscillator 11 and output the first and second IF signals IIF and QIF by mixing the RF input signal with the first and second LO signals LOI and LOQ, respectively, and the first and second IF signals IIF and QIF should be different in phase by 90 degrees.
FIGS. 2A and 2B are block diagrams of the I-mixer 13 and the Q-mixer 14, respectively. Referring to FIGS. 2A and 2B, the I-mixer 13 includes a first RF amplifier the RF input signal RFIN and converting voltage to current, a first switching unit 13b on-off switching an output of the first RF amplifier 13a according to the LO signal (LOI or LOQ), and a first load unit 13c converting an output of the first switching unit 13b into a voltage signal (IIF). The Q mixer 14 includes a second RF amplifier 14a amplifying the RF input signal RFIN and converting voltage current, a second switching unit 14b on-off switching an output of the second RF amplifier 14a according to the LO signal (LOI or LOQ), and a second load unit 14c converting an output of the second switching unit 14b into a voltage signal (QIF).
However, the first LO signal LOI and the second LO signal LOQ cannot form a 90 degree difference in phase if components associated with local oscillator 11 lose symmetry in phase. As a result, the first IF signal IIF and the second IF signal QIF cannot form the 90 degree difference in phase. That is, a miss-matching error between the first LO signal LOI and the second LO signal LOQ causes another miss-matching error between the first IF signal IIF and the second IF signal QIF.
FIG. 3 is a block diagram of another conventional frequency conversion apparatus. Referring to FIG. 3, the frequency conversion apparatus includes a quadrature signal generator QSG 31 outputting two oscillating frequency (LO) signals having the same frequency and a 90 degree phase difference, mixers 33, 34 mixing an RF input signal with the LO signals of the QSG 31, filters 35,36 coupled to the corresponding mixers 33, 34 and a phase detector 37 providing phase control signals C1, C2 to the filters 35, 36 and the QSG 31 to eliminate a miss-matching error between the LO signals.
The frequency convention apparatus shown in FIG. 3 uses a phase miss-matching eliminating method of detecting a phase difference corresponding to the phase miss-matching error between the first and second IF signals IF1, IF2 and providing the phase difference to the filters 35, 36 and the QSG 35 to eliminate the miss-matching error of the first and second IF signals IF1, IF2.
However, this conventional frequency conversion apparatus cannot be used in the image-rejection receiving structure since the image-rejection ratio is determined according to the phase miss-matching between the first and second IF signals IF1, IF2 or the LO signals (LOI, LOQ) in the image-rejection receiving structure, and a structure and a size of the image-rejection receiving structure become bulky due to complexity of the image-rejection receiving structure implemented with the conventional frequency conversion apparatus.